Modelling of Moisture Adsorption for Sugar Palm (Arenga pinnata) Starch Film

Tri Hadi Jatmiko ^a) , Crescentiana D. Poeloengasih, Dwi Joko Prasetyo and Hernawan

Research Unit for Natural Product Technology, Indonesian Institute of Sciences, Gunungkidul, Yogyakarta, Indonesia

Abstract. Sorption characteristic of food products is important for design, optimization, storage and modelling. Sugar palm starch film with two different plasticizers (sorbitol and glycerol) with varied concentration studied for its adsorption isotherm characteristic. The data of adsorption isotherm fitted with GAB, Oswin, Smith and Peleg models. All models describe the experiment data well, but Peleg model is better than the other models on both sugar palm starch film plasticized with sorbitol and glycerol. Moisture sorption of sugar palm starch increased linearly with plasticizer concentration. A new model by taking account of plasticizer concentration describes the experiment data well with an average of coefficients of determination (R²) 0.9913 and 0.9939 for film plasticized with glycerol and sorbitol respectively.

Keywords: Sugar palm starch; glycerol; sorbitol; model; moisture sorption isotherm

Utilization of biopolymers for the film has attracted the interest of researchers to explore starch as a material for the film. Starch has attracted great attention because it is easily obtained, widely available, cheap, eco-friendly, renewable and film-forming properties [1,2]. The studies that have been conducted shows that the sugar palm starch has the potential to be used as raw material for the film [1,3-6]. The use of pure starch will produce the fragile film, which is usually overcome by the addition of plasticizers. The addition of plasticizer on the film made of starch will affect the characteristics of the film, one that has changed is the characteristic of moisture absorption of the film.

Moisture sorption isotherm is the relationship between the amount of water content of foodstuffs with humidity at a constant temperature and displayed in graphical form [7]. Moisture sorption isotherm models are useful for predicting water sorption characteristics of foods, even though they furnish little insight into the interaction of water and food. Even though a number of mathematical models exist to explain moisture sorption isotherms of foods substances, none equation offers accurate outcome for the period of the entire variety of water activities, or for all types of foods material, it is because of water associated with a matrix of food with different mechanisms on different humidity [7].

Only a few studies have reported the content of the plasticizer into account in the determination of moisture sorption isotherm of a starch film. Coupland (2000) reported the effect of glycerol on the moisture sorption behavior of whey protein isolate film, that consider the content of plasticizer in moister sorption of the film [8]. Jatmiko (2016) reported that four parameter Peleg model could be used to give a good description of moisture sorption of sugar palm starch based film. In this study, we report sorption isotherms for sugar palm starch based films affected by plasticizer.

Moisture sorption isotherm of sugar palm starch film from Jatmiko (2016) was used for this study. The data fitted with the following model:

Oswin

Oswin equation is one of the best model for describing the moisture sorption of starchy food and gave a good fit for vegetables and meat [7].

where M is the moisture content (g/g dry solid), a_w is water activity, A and B are constant.

Smith

Smith has developed a water sorption isotherm equation based on the theory that water adsorbed on a dry surface composed of two fractions. The first fraction has a heat of condensation is higher than normal and the second fraction consisting of multilayers of water molecules, which can prevent the evaporation of the initial layer [9].

where M is the moisture content (g/g dry solid), A is the quantity of water in the first sorbed fraction, and B is the quantity of water in the multilayer moisture fraction, a_w is water activity.

GAB

GAB equation is one of the most widely used equations in predicting water sorption isotherms [7].

where M is the moisture content (g/g dry solid), M₀ is the monolayer moisture content; C and K are constants.

Peleg

Four parameters model proposed by Peleg [10] can be used for both sigmoid and non-sigmoid isotherm and some studies report that Peleg model better than GAB model.

where M is the moisture content (g/g dry solid), K₁, K₂, n₁ and n₂ are constants.

Moisture sorption of sugar palm starch film with sorbitol and glycerol shows sigmoidal shape as shown in Fig. 1. According to the classification of Al-Muhtaseb et. al [7] the moisture sorption of sugar palm starch film is type III.

FIGURE 1. Moisture Sorption isotherm of sugar palm starch film plasticized with sorbitol (A) and glycerol (B)

The data of moisture sorption of sugar palm starch film with glycerol and sorbitol plasticizer were fitted with models from previous studies. Generally, all models describe moisture sorption isotherm of sugar palm starch film plasticized with glycerol and sorbitol well. The model constants from previous studies present in Table 1 and Table 2.

TABLE 1. Model constants for sugar palm starch film with glycerol

TABLE 2. Model constants for sugar palm starch film with sorbitol

GAB equation shows that the higher the concentration of plasticizer, the amount of water in the monolayer will be even greater. According to Mali [11], this happens because the more content of the plasticizer, the more active sites that bind water. The moisture content on a monolayer of sugar palm starch film plasticized with sorbitol was lower than sugar palm starch film plasticized with glycerol. Sorbitol structural molecule similar to glucose that cause strong interaction between sorbitol and polymer chain, as a result, there is a lower possibility for sorbitol to interact with water [12]. Meanwhile, glycerol could withstand water in their matrix because the hydroxyl group in glycerol had a strong affinity with water [13].

All of the above models can describe the moisture sorption isotherms by the film of sugar palm starch well, but none of them describe the effect of the concentration of plasticizer in moisture sorption isotherms by sugar palm starch film. So we proposed a new model that consider the concentration of plasticizer on moisture sorption of sugar palm starch film plasticized with glycerol and sorbitol.

where M is the moisture content (g/g dry solid), a, b, c, d constant and x is plasticizer concentration.

TABLE 3. Constants of new model for sugar palm starch film

Table 3. shows the model constants and coefficient of determination that describe the moisture sorption of sugar palm starch film plasticized with sorbitol and glycerol well.

The moisture sorption isotherm of sugar palm starch film increase linearly with plasticizer concentration. A new model that consider the plasticizer content show the sorption isotherm sugar palm starch film well.

The authors grateful to Indonesian Institute of Sciences for providing financial assistance through Riset Unggulan 2016 during this investigation. We also extent our appreciation to Deputy of Engineering Science, Indonesian Institute of Sciences for his encouragement and support during this work

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